Children from several study areas will be evaluated to determine the etiology of acute diarrhea, including Hermann Hospital Outpatient Clinic (Houston), San Jose Clinic (Houston) and Roosevelt Hospital Clinic (Guatemala). A control population without illness will be studied from among infants and children at Hermann Hospital Well Baby and Pediatric Clinics. Salmonella, Shigella, enteropathogenic Escherichia coli (EPEC) and parasites will be determined by conventional techniques. Pathogenicity of bacterial isolates (E. coli and other facultative organisms) obtained from stool will be examined in a variety of tissue culture (adrenal cells, HeLa cells) and animal models (rabbit loop, skin permeability test). Stool filtrates will be studied by electron microscopy for viral particles both by direct examination and following incubation of filtrates with serum obtained from the patient (immune electron microscopy). A healthy control population will be studied for presence of enteropathogens for comparison. Acute and convalescent serum samples will be obtained on all patients with diarrhea. In children in whom a toxigenic E. coli is isolated serum toxin-neutralizing antibodies will be measured as well as antibodies directed against somatic antigens of the organism. When viral particles are found by electron microscopy, serum titers of antibody and cross antigenicity of visualized viruses will be determined by immune electron microscopy. The three different populations studied will allow a close examination of differing epidemiologic pattern and relation of nutritional and socioeconomic status to specific etiology.